FIG. 1 shows a prior art charge or current pump for providing a current to load Z through resistor R1. In this prior art device, a switching circuit SW couples an input of load Z to either a plus voltage +V at terminal 1, a negative voltage -V at terminal 2, or an open circuit terminal 3. Thus, when switch SW is coupled to terminal 1, a positive current will flow through resistor R1 while, when switch SW is connected to terminal 2, a negative current will flow through resistor R1. When switch SW is coupled to terminal 3, ideally no current will flow through resistor R1.
An alternative prior art current pump is shown in FIG. 2 where switch SW couples an input of load Z to either a positive current source +I at terminal 1, a negative current source -I at terminal 2, or an open circuit terminal 3. When switch SW is coupled to terminal 3, ideally no current will flow through load Z.
FIG. 3 shows a prior art current pump circuit which provides to a load connected to node 1 either a high voltage, a low voltage, or a high impedance. If a control signal applied to terminal PD is low while a control signal applied to terminal PU is high, a high voltage will appear at node 1, and a positive current will flow from node 1 to a load. By applying a low voltage to the circuit at input terminal PD, transistor Q1 will turn on and pull the base of transistor Q2 down, preventing transistor Q2 from turning on. Transistor Q3 will then only turn partially on and not fully on in order to maintain a voltage at its collector sufficient to turn transistor Q4 on and provide a current across resistors R1 and R2 to supply the necessary bias voltage to the base of transistor Q3. The high voltage at the emitter of transistor Q4 will cause a high voltage to appear at node 1. If the voltage applied to input terminal PU is high at this time, diode D1 would be reversed biased and act as an open circuit. Thus, with PD low and PU high, a positive current will flow to a load from node 1.
Alternatively, to provide a negative current at node 1, a low voltage is applied to terminal PU so as to cause the voltage at the emitter of transistor Q5 to be clamped at a maximum of one diode drop above the low voltage at terminal PU. This insures that transistor Q3 cannot be turned on since its base voltage is necessarily clamped at one diode drop above ground due to the voltage at the emitter of transistor Q5 being clamped at one diode drop above ground. If the voltage applied to terminal PD is high, transistor Q1 will conduct in the reverse direction and turn transistor Q2 fully on. This will render transistors Q4 and Q5 nonconductive. Thus the voltage at node 1 will be low, and the output current will be negative.
For an effective open circuit at node 1, input voltages applied to terminals PD and PU are made high so that transistor Q5 is off and diode D1 is off, causing node 1 to be isolated.
For a current pump of the type shown in FIG. 1 to operate as a constant current pump, load Z must be of a very low impedance compared to R1. This will make the node that load Z is attached to appear as a virtual ground. Hence, when the switch is connected to either voltage source, there is a constant voltage across R and therefore a constant current. However, frequently load Z is not of a low impedance and may in fact have a varying impedance. Thus, the current pumped may not be constant or well-defined.
Another drawback to the type of current pump shown in FIG. 1 is the difficulty in real circuits of isolating the unconnected terminals of the switch from the output node. For example, if the switch is connected to terminal 3, there may be leakage onto the output node from the unconnected terminals 1 and 2.
Current pumps of the type shown in FIG. 2 also have this difficulty in isolating unconnected terminals. A disadvantage, in some implementations, of the circuit of FIG. 2 is that an unconnected current source can drift to an unknown and unwanted voltage. This can cause troublesome transients when that terminal is connected back to the load.
One of the advantages of the type of current pumps shown in FIG. 2 over the type of current pumps shown in FIG. 1 is its immunity to noise on the power supply line. Since the voltage sources used in the FIG. 1 type of circuits are low impedance, any noise on the power supply line can be easily transmitted through the closed switch to the output terminal. The FIG. 2 type pumps use current sources, which are inherently high impedance, therefore much more immune to noise on the power supply line.
Further, since in operation, a current pump output is applied intermittently to a load for fixed durations, charge supplied during a positive current pulse ideally should negate the charge removed during a negative current pulse of the same duration. It is difficult to precisely set the high voltage and low voltage output levels of the current pump to that necessary to cause a positive current pulse to transfer the same charge as a negative current pulse having a same duration.